Fabrication of semiconductor integrated circuits involves, among other things, repeated application of four basic fabrication steps: masking, etching, layer formation through deposition or growth, and doping. Presently, etching to remove part or all of a layer of semiconductor material is performed by wet etching, using chemicals, and by dry etching using ions, a plasma or reactive dry chemistry. Dry etching has developed most recently and is especially favored where sharply defined, anisotropic profiles are required. However, even where a dry etchant with high selectivity is used to remove a given layer, some of the underlying material can be etched away in the process, which is often undesirable. As the characteristic sizes of integrated circuit geometries are reduced below 1 micron, some of the scaled structural features of such circuits become highly sensitive to even a small amount of overetching or excessive growth. Similarly, the thickness of a grown layer of semiconductor material must be controlled closely if threshold voltages and other circuit parameters are to be prescribed with any degree of confidence in these micron-size or sub-micron-size circuit geometries. Thus, accurate monitoring of the present thickness of a layer of semiconductor material that is being etched or grown, and unambiguous identification of the end point of such a process, is important for semiconductor fabrication.
One approach to end point detection of an etching process requires monitoring the composition of gas adjacent to the etched layer for the absence of, or substantially reduced concentration of, the etched material or its known compounds, through laser-induced fluorescence or a similar process. The end point of this process occurs when the layer is completely etched through.
Another approach to etching end point detection requires monitoring the visible light emitted by a plasma at a predetermined wavelength, such as a characteristic emission line for the etched material or its compounds. Etching is terminated when the intensity of the characteristic emission line is reduced, indicating the absence or reduced concentration of the etched material, or increased abundance of reactants, or presence of new reactants whose source is the layer beneath the layer of material being monitored.
A third technique for end point detection uses a laser interferometer and attempts to directly monitor the present depth of the layer being etched or grown. This method has some associated ambiguities in determination of the depth, because several different depths of the material of interest may produce the same interferometric signal, as noted by M. Born and E. Wolf in "Principles of Optics", Pergamon Press, 5th Edition, 1975, p. 62.
Maydan et al., in U.S. Pat. No. 4,618,262, discloses the use of a laser interferometer system to detect the end point of an etch process and to monitor the present depth of a layer undergoing etching or growth. This process requires that the laser beam first be scanned across scribe lines on a wafer that is undergoing fabrication, locating an appropriate region adjacent to or within a scribe line, and monitoring the interferometric signal produced when the laser is directed at and allowed to reflect from that area on the wafer surface. The Maydan et al. invention monitors the interference pattern of repetitive maxima and minima produced in an interference pattern and requires that the laser beam be focused on a region of the semiconductor surface where optically degrading structural features, such as trenches and other sharp changes in the side profile of a semiconductor wafer or chip, are absent.
An interferometric system for measuring both etch rate and etch depth of a semiconductor body is disclosed by Muething in U.S. Pat. No. 4,660,979, issued on "Method and Apparatus for Automatically Measuring Semiconductor Process Parameters." The interferometer directs light toward the semiconductor body and detects the intensity of reflected light; this intensity will vary periodically during the etching process. A counter circuit responds to the periodically varying intensity of the reflected light and determines the number of cycles and the present frequency of variation thereof to compute present etch rate and etch depth in the semiconductor body.
The "Interferometric Methods and Apparatus for Device Fabrication" disclosed by Heimann et al. in U.S. Pat. No. 4,680,084 monitors etch depth by monitoring the intensity of monochromatic light reflected from the active surface of the body being etched. The wavelength of light used for reflection is chosen so that a first, exposed overlying portion of the semiconductor body is substantially transparent at that wavelength and an underlying portion of the semiconductor body which is not to be etched, is substantially opaque at that wavelength.
Several problems are manifest in one or more of the approaches in the prior art. First, scanning of the wafer surface is required, which requires special apparatus and use of a criterion that determines whether a suitable measurement location, such as a scribe line, is available. This requires complex analysis of the surface reflection signal, and the completion of the entire procedure can consume a considerable period of time. Second, measurement of both thick and thin films with the same process is difficult or impossible because of the assumptions often made in the thickness analysis. Third, access to a suitable spot, such as a scribe line or exposed portion of the surface whose growth or removal is to be monitored, is not guaranteed. If no suitable spot is found, another approach must be sought and used for monitoring that particular growth or removal process. Thus, different monitoring techniques might be required for monitoring different stages of the fabrication process.
What is needed is an optical monitoring system that determines changes in thickness of a layer of semiconductor or other material that is being etched or grown, as a function of time, which is not subject to ambiguities in the optical parameters thereby determined and that does not require scanning of the light beam across scribe lines or other features of the material surface.